1. Field of the Invention
This disclosure relates to an image forming apparatuses such as copying machines, printers, facsimile machines, and multifunction peripherals configured to form images on the basis of an electrophotographic image forming system or an electrostatic recording method.
2. Description of the Related Art
Examples of the image forming apparatuses of the electrophotographic image forming system include an in-line type image forming apparatus having a plurality of photosensitive members and process units (charging units, developing units, and cleaning units) operated in conjunction with the photosensitive members, and a belt configured to come into contact with the photosensitive members, and configured to be capable of forming color images on a transfer material.
There is also an image forming apparatus employing a contact developing method which performs developing in a state in which developing rollers are in contact with a photosensitive members. When employing the contact developing method, phenomena such as shortening of the lifetime caused by wearing of a surface layer of the photosensitive member due to sliding contact with the developing roller, waste of developer and contamination of the transfer material caused by the developer (toner) adhered to the photosensitive member at the time other than image formation, and deformation of the developing roller by being kept in a stopped state in contact for a long time may occur.
In Japanese Patent No. 4667106, a configuration in which an occurrence of the above-described phenomena is suppressed when the contact developing method is employed in the in-line type image forming apparatus is proposed. Specifically, the developing roller is configured to be movable between an contact position in contact with the photosensitive member and a separated position separated from the photosensitive member, the developing roller is arranged at the contact position with respect to the photosensitive member only during a period in which the electrostatic latent image on the photosensitive member is developed, and is arranged at the separated position during other periods.
When performing color image formation with the in-line configuration, the image formation is started on the respective photosensitive members in sequence with time lags in conformity with a rotation of a belt so that toner images transferred from the respective photosensitive members are overlapped one on top of another on the belt of on a transfer material conveyed by the belt. In contrast, in Japanese Patent No. 4667106, the plurality of developing rollers are configured to come into contact with corresponding photosensitive members with time lag in sequence so as to keep the developing rollers separated from the photosensitive member as long as possible until immediately before starting development.
Here, the developing rollers each need to be moved slowly from the separated position to the contact position in a predetermined period so as to avoid a distortion of images due to shaking of the apparatus caused by an impact caused by the developing roller coming into contact with the photosensitive drums.
On the other hand, in order to improve the usability, shortening of a time period from an input of a print signal to the image forming apparatus until an output of a first transfer material with a toner image formed thereon (FPOT=First Print Out Time) is required in recent years. One of conceivable methods to reduce the FPOT is shortening the time period from the input of the print signal to the image forming apparatus until the start of development for the first time. Therefore, a method of shortening the FPOT by shortening the time period required for the developing roller which starts development firstly to move to a contact position is conceivable.
However, in Japanese Patent No. 4667106, the time periods required for moving the respective developing rollers from the separated positions to the contact positions are set to be all the same. Therefore, when the time period required for moving each of the developing rollers from the separated position to the contact position is set to suppress the impact occurring when the developing roller comes into contact with the photosensitive drum, a time period required for the developing roller which firstly starts the development to move from the separated position to the contact position may become an obstacle for shortening the FPOT.
In other words, in the configuration disclosed in Japanese Patent No. 4667106, when an attempt is made to shorten the time period required for the developing roller which firstly starts the development to move from the separated position to the contact position in order to shorter the FPOT, the time periods required for other developing rollers to move from the separated positions to the contact positions are also shortened, so that the image may be distorted due to the impact occurring when other developing rollers come into contact with the photosensitive drums.